Every large-scale energy source comes with a downside. For renewables, it's intermittency, the inability to produce power when the sun isn't shining and the wind is still. Many experts worry that the steadily climbing share of electricity from renewable sources will eventually make portions of the electric grid unstable. Researchers working on the cutting edge of battery technology, however, are increasingly confident that they are on track to solve the problem. They're updating a 130-year-old technology called flow batteries, aiming to create versions that can bank massive amounts of electricity from renewables when it's not needed and then feed it back into the grid during times of heavy demand.